This invention relates to in-situ remediation technology using zero valent iron powder for cleanup of industrially generated toxic heavy metal cations and halogenated hydrocarbons in subsurface geological formations.
Industrial manufacturing activities concerning inorganic chemicals, batteries, coil coating, leather tanning and finishing, electrical/electronic components, foundries, iron and steel, photographic equipment and supplies, metal plating and finishing, non-ferrous metals, mining, paint and ink formulators employ raw materials and produce wastes that contain such heavy metals as cadmium, chromium, lead, nickel, and copper. These industries also employ fire resistant halogenated solvents such as trichloroethylene or tetrachioroethylene for cleaning and degreasing operations. Spills, leaking storage tanks and pipe lines, or improper disposal of waste containing these hazardous materials can lead to subsurface soil and groundwater contamination. This occurs as a result of these chemicals migrating into the subsurface soil and groundwater with infiltrating rainfall. Exposure to these contaminants in subsurface soils and groundwater can pose a serious hazard to human health.
The following technology is currently being used for remediation of subsurface toxic heavy metal cations and halogenated hydrocarbons in geological formations.
Remediation of subsurface toxic heavy metal cations and halogenated hydrocarbons present in saturated soils and groundwater can be achieved by transferring the dissolved heavy metal cations and anions and halogenated hydrocarbon present in groundwater to the surface for treatment using conventional chemical and physical processes. The mobility of the non-soluble or undissolved phase heavy metal cations and anions and halogenated hydrocarbons can be increased for improved recovery through addition of surfactant and chelating agents to increase their solubility in the groundwater.
Improvements in technologies for use in geological formations that are dense and exhibit low permeability and hydraulic conductivity have been accomplished through the use of pneumatic fracturing using compressed gases, as shown in U.S. Pat. No. 5,032,042, the subject matter of which is incorporated herein by reference, and hydraulic fracturing using liquid and a fracture maintaining or proppant material as the injection fluid. These technologies create fractures or preferential flow channels within low permeability geological formations through which volatilized and dissolved halogenated hydrocarbons can be removed for surface treatment.
However, there exist several inherent disadvantages associated with the use of these technologies which require above-ground treatment. Since above-ground surface structures and treatment systems must be erected on site to treat the recovered extracted vapors or liquids from the subsurface formations, considerable costs associated with installation and operation can result. Also, the presence of these above-ground structures and treatment systems limit the use and accessibility of the property during the remediation operations.
Remediation technologies that are applied in-situ can overcome the disadvantages associated with the above-ground treatment systems.
In-situ bioremediation uses indigenous and/or mixed proprietary microorganism supplied with needed nutrients, moisture, and environment to biodegrade in-situ the halogenated hydrocarbons with desirable rates. Microorganism growth within the subsurface contaminated zones can be achieved by supplying additional microorganisms, nutrients, and moisture, altering environmental conditions (pH, etc.) and atmosphere through injection of air oxygen microbubbles, nitrogen, etc. In-situ delivery systems such as gas injection, hydraulic injection or hollow shaft auger drills with extended nozzles have been used creating in-situ environments for the microbial activity to flourish.
There exist several disadvantages associated with the use of in-situ bioremediation of halogenated hydrocarbons. Only low levels of soluble halogenated hydrocarbon can be remediated, since high concentrations of hydrocarbons can be toxic to the microorganisms. For example, microorganisms will not completely remove the chlorine atom from trichloroethylene if its concentration exceeds 30 parts per million (ppm) in water. Also, in-situ biodegradation of halogenated hydrocarbons is difficult to achieve because a combination of anaerobic (metabolism in absence of oxygen) and aerobic (metabolism requires oxygen) microorganisms are needed to degrade these organic compounds. Also, microbial clogging can occur around boreholes, which reduces the ability of the delivery system to provide needed nutrients and create an environment required to maintain subsurface microbial activity.
Additionally, treatment of soluble heavy metal ions and halogenated hydrocarbons in groundwater can be carried out using iron powder, granular iron metal filings or iron chips. All of these technologies rely on the use of beds or reactive walls containing large quantities of permanent or replaceable iron located solely within the saturated zone of a geological formation and where treatment occurs solely downstream of the ground water passing through sources of upstream heavy metal cations and halogenated hydrocarbons upstream in the formation. Large quantities of iron are used to insure that the contaminated groundwater flowing through the iron is remediated within the time required for the contaminated groundwater to pass through the bed or reactive wall.
Typically, the bed or reactive wall is designed so that it creates a more permeable flow area within the contaminated groundwater flow pattern. As the contaminated groundwater flows through the bed of iron filings, contact between the iron and the soluble contaminants results in dechlorination of the halogenated hydrocarbons and the reduction of the heavy metal ions to significantly less hazardous forms. Some of these technologies renew the iron bed material periodically to maintain its reactivity in reducing the heavy metal ions and halogenated hydrocarbons.
Although the iron bed or reaction wall methods are effective in reducing toxic heavy metal cations more noble than iron and halogenated hydrocarbons to desired levels in the ground water, the processes do not provide any treatment of the non-soluble phase contamination that may exist in the geological formation. Also, the contaminants in the soil being treated must be included within the ground water flowing through the permeable ion treatment beds or wall to be treated. In geological formations that exhibit complex groundwater flows, placement of the beds or reactive walls to intercept all contaminated groundwater flow is difficult to achieve. Additionally, the practice of utilizing large quantities of iron gives rise to significant reduction of ground water which is accompanied by the undesirable production of excess quantities of hydrogen that could lead to hazardous conditions in confined environments.